CAST IRONA large family of alloys, generally containing morethan 2% carbon and between 1% and 3% silicon.

Unlike steels, they are not malleable when solid, andmost have low ductility and very poor resistance toimpact loading. However, cast irons are very usefulwhen intricate or inexpensive castings are required,and they provide a high damping capacity (the abilityof a material to absorb vibration) which can be importantfor precision machinery. Cast irons have a lowmelting temperature, are very fluid when molten, andshrink very little during solidification.Unlike steels, cast irons contain free graphitegrains, and it is the shape and distribution of the freegraphite grains which have the strongest effect on theproperties of the cast iron. Also important is the matrixin which they occur. The microstructure of the matricesdepends on the alloys present in the metal, and therate at which it solidifies and cools. If this sequence isvery rapid, the dissolved carbon does not have enoughtime to nucleate as graphite during solidification;while the matrix transforms to harder microstructure.Subsequent heat treatments are also important to temperthe very hard structures.Silicon is added to cast iron primarily to control thesolubility of carbon, and therefore the characteristicsof the graphite. Additionally, silicon serves as a deoxidizer,promotes fluidity, and decreases shrinkage. Sulfurmight be present in the alloy but is not addedintentionally, since it causes hot cracks and can produceporosity if present in high concentrations. Phosphorousis also undesirable, because it produces ahard, brittle compound; its low melting temperaturecontributes to hot-cracking problems. However, phosphorousincreases the fluidity of these irons, which is adesirable characteristic when casting very thin sections.Manganese is added to tie up the sulphur as ahigh-melting compound in order to reduce the problemof hot cracking. Manganese is also used to controlthe microstructure of welds, improving the strengthand ductility as well as machinability.Types of Cast Irons

The four basic types of cast iron are gray, white,ductile, and malleable.The gray cast irons contain flake graphite, whichimparts a gray surface in fractures, and are the mostcommon of the cast irons. The gray irons are readilymachinable.The white cast irons exhibit crystalline, whitishfractures because the carbon remains in solution duringsolidification, producing massive carbides in apearlitic matrix. They are very brittle and hard, butvery wear-resistant.The ductile irons are also known as nodular irons.They contain alloys which cause the graphite to nucleateas spheres. These nodules are encased in a layer offerrite and are in a pearlitic matrix, making them veryductile. Some nodules exhibit elongations of up to18%.Malleable cast irons are produced by heat treatingspecially alloyed white cast irons. Heat treating resultsin the development of graphite nodules (temper carbon)in a ferrite matrix. Malleable cast irons are usedwhen good strength, toughness, and casting andmachining properties are required.Welding ConsiderationsBecause of the considerable differences in the compositionand microstructure of the cast irons, it isessential to identify the type of cast iron before weldingbegins. Some insight about the type of iron can bedetermined from the appearance of fractures, an examinationof the microstructure, or from hardness measurements.A chemical analysis would also be helpful.When no information is obtainable, the iron could beassumed to be gray cast iron because of its general use,and the procedures selected on that basis.The selection of the filler metal, the energy input,and the preheat are very important to successful welding. Selection of the welding process is also importantin establishing the procedures and materials to beused.

When welding cast iron with a steel electrode, thereare four important zones in the vicinity of the weld:the weld metal area, the alloyed weld metal zone, theheat-affected zone, and the original cast iron. See FigureC-1. In C-1, Section A is deposited steel, unaffectedby dilution. Section B is also steel depositedfrom the electrode, but changed from a soft steel to arather high-carbon steel. This is due to alloying withcarbon from the cast iron.

Cast iron which was brought up to melting temperature,then chilled by the cold mass of the casting, isshown as Section C, the heat-affected zone. The resultis a metal which is extremely hard and brittle. Thishardening is due to rapid cooling, which prevents theiron carbide from changing into iron and graphite. Themetal in this area is white iron; the controlling componentis cementite.Section D is the original cast iron.Failure of a welded joint of the type shown in Figure

C-1 will usually occur in the cast iron adjacent tothe line of fusion, because the hardened cast iron ismore brittle than the high-carbon steel on the steel sideof the line of fusion.